JPH1053421A - Powder for lower layer of coating type magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a nonmagnetic layer containing iron oxyhydroxide powder comprising acicular particles having a specific average major axis length and releasing H2 O in a specified amount or less on heating, and having excellent surface smoothness. SOLUTION: A suspension containing ferrous hydroxide colloid obtained by the addition of an aqueous alkali hydroxide solution to an aqueous ferrous salt solution is oxidized by the aeration of an oxygen-containing gas at a pH of approximately >=11 at a temperature of <=80 deg.C to obtain acicular iron oxyhydroxide powder having an average major axis length of 0.01-0.5μm, an average minor axis length of 0.01-0.05μm, an average axis ratio of 1-30, a specific surface area of 10-300m<2> /g, a crystal particle diameter of 10-200Å and a releasing water amount of <=2wt.% at 100 deg.C. The iron oxyhydroxide powder or the iron oxyhydroxide powder coated with 0.1-30wt.% of Al and/or Si is dispersed in a resin binder comprising a urethane resin and methyl ethyl ketone/ cyclohexanone/toluene, etc., and subsequently coated on a base film such as a polyethylene terephthalate film to form a nonmagnetic lower layer for a magnetic recording medium.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lower layer powder used for a coating type magnetic recording medium having a multilayer structure.

[0002]

2. Description of the Related Art A so-called coating type magnetic recording medium in which a magnetic layer is formed on a support by applying a coating film in which magnetic powder is dispersed and contained in a binder resin (binder) is formed on the support, has low noise. In order to obtain high output characteristics, it is desired to reduce the thickness of the magnetic layer. For this purpose, a non-magnetic powder in which a non-magnetic powder is dispersed and contained in a binder resin is interposed between the magnetic layer and the support. Layer coating (herein referred to as the lower layer)
There has been proposed a coating type magnetic recording medium having a multilayer structure for forming a magnetic recording medium.

Heretofore, spherical titanium oxide powder or acicular iron oxide powder has been mainly used as a nonmagnetic powder for forming the lower layer. A magnetic recording medium having such a lower layer is disclosed in, for example, JP-A-63-18741.
No. 8, JP-A-4-167225, JP-A-6-167225
No. 60362 and Japanese Patent Application Laid-Open No. 6-131653. In addition, Japanese Patent Laid-Open No. 4-1672
No. 25, JP-A-6-139553, JP-A-6-139553
-215360, JP-A-7-78331,
JP-A-7-105530, JP-A-7-18264
9, JP-A-7-282443, JP-A-7-282443
Japanese Patent Application Laid-Open No. 326037, Japanese Patent Application Laid-Open No. Hei 7-334835, and the like show characteristic values of such a magnetic recording medium having a multilayer structure when needle-like hematite or the like is used as a nonmagnetic powder for forming a lower layer. It is also taught that iron oxyhydroxide and the like can be used as the needle-shaped nonmagnetic powder in addition to the disclosed embodiment.

[0004]

However, there has been no record of using iron oxyhydroxide as a lower layer powder in a magnetic recording medium having a multilayer structure, and in the above-mentioned publication, iron oxyhydroxide (FeOOH) was used for the lower layer. A specific example in the case of powder is not shown. Therefore, it is often unknown what kind of iron oxyhydroxide exhibits the intended function as the lower layer powder for the magnetic recording medium. On the other hand, iron oxyhydroxide is generally produced by a method of oxidizing a suspension of Fe (OH) _{2. As} is well known, even if the conditions of this oxidation are slightly changed, the formed phases are different. , And have different properties and forms. Therefore, not all known iron oxyhydroxides have properties suitable for the above-mentioned lower layer powder.

According to the present invention, when the iron oxyhydroxide powder is applied to the powder for the lower layer, how the chemical / physical properties and shape characteristics of the powder are adjusted to determine the surface smoothness, strength,
It is an object of the present invention to clarify whether magnetic characteristics and weather resistance are affected, and to contribute to improving the characteristics of a multilayered magnetic recording medium.

[0006]

According to the present invention, there are provided needle-like particles having an average major axis length of 0.01 to 0.5 μm, and
Provided is a lower layer powder for a coating type magnetic recording medium comprising an iron oxyhydroxide powder having an amount of H ₂ O released at 0 ° C. of 2% by weight or less. Further, according to the present invention, the amount of H ₂ O composed of needle-like particles having an average major axis length of 0.01 to 0.5 μm having a branching direction biased in a two-dimensional direction and released at 100 ° C. is 2% by weight. A lower layer powder for a coating type magnetic recording medium comprising the following iron oxyhydroxide powder is provided.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The primary purpose of providing a non-magnetic layer (lower layer) in which non-magnetic powder is dispersed between a support and a magnetic layer is to reduce the thickness of the magnetic layer so that it can be used in a short recording wavelength region. And to improve excellent electromagnetic conversion characteristics such as erasing characteristics and overwriting characteristics. For this purpose, the magnetic layer itself is required to have a certain characteristic, but the role of the lower nonmagnetic layer side is that a smooth thin magnetic layer with little surface irregularities can be applied thereon, that is, the nonmagnetic layer Mainly, they are excellent in surface smoothness themselves, contribute to the strength of the magnetic recording medium, and can sufficiently bring out the magnetic properties of the upper magnetic layer.

[0008] Spherical titanium oxide which has been used as the powder for the lower layer, when formed into a tape, does not have sufficient strength as compared with a needle-shaped powder, and it is difficult to form fine particles. In addition, acicular iron oxide (hematite) has a problem in that surface sintering cannot be sufficiently obtained because sintering between particles cannot be avoided due to its manufacturing method.

When forming a coating film in which iron oxyhydroxide is dispersed in a binder resin, the surface smoothness and strength depend on the binder resin used, but the physical and chemical properties of the iron oxyhydroxide And size and shape are greatly affected. The iron oxyhydroxide powder for the lower layer capable of fulfilling the role of the lower layer, that is, improving the surface smoothness, strength and properties of the magnetic layer, is composed of acicular particles having an average major axis length of 0.01 to 0.5 μm. It is also preferable that the amount of H ₂ O released at 100 ° C. be 2% by weight or less.

Further, the role that the lower layer should have is that the branching direction has a two-dimensionally biased average long axis length of 0.0.
By of H ₂ O amount is 2% by weight of iron oxyhydroxide powder that releases at and 100 ° C. consists acicular particles of 1～0.5Myuemu, can play more advantageously.

The above-mentioned role is that the average major axis length is 0.01-0.
Consisting of 5 μm needle-shaped particles, 0.1 to 30% by weight of Al
Of H ₂ O containing and releasing at 100 ° C. is 2% by weight
It can be more advantageously achieved by the following iron oxyhydroxide powder.

The above-mentioned role is that the average major axis length is 0.01-0.
Consisting of 5 μm needle-like particles, 0.1 to 30% by weight of Si
Of H ₂ O containing and releasing at 100 ° C. is 2% by weight
It can be more advantageously achieved by the following iron oxyhydroxide powder.

The above-mentioned role is that the average major axis length is 0.01-0.
Consisting of 5 μm needle-shaped particles, Al and Si were added in a total of 0.1
By of H ₂ O amount is 2% by weight of iron oxyhydroxide powder emitting at 30% content by and 100 ° C., it can be accomplished more advantageously.

The above-mentioned role is that the average major axis length is 0.01-0.
This can be achieved more advantageously by iron oxyhydroxide powder comprising needle-like particles of 5 μm, having a tap density of 0.4 or more and releasing H ₂ O at 100 ° C. of 2% by weight or less.

Further, the above-mentioned role is that the average major axis length is 0.01.
Consisting of needle-like particles of 0.5 to 0.5 μm, 0.1 to 30% by weight
H ₂ containing Al at a decomposition temperature in the air of 210 ° C. or higher, preferably 215 ° C. or higher and released at 100 ° C.
Even with iron oxyhydroxide powder having an O content of 2% by weight or less,
It can be performed more advantageously. The iron oxyhydroxide powder according to the present invention preferably has the following characteristics in addition to the above.

[Specific surface area] As measured by the BET method, 1
It may be in the range of 0 to 300 m ² / g, preferably 40 m ² / g
/ g or more, more preferably 40 to 150 m ² / g. [Tap density] 0.3 to 0.8 g / cm ³ , preferably 0.3 g / cm ³
It is preferably 40 g / cm ³ or more. [Compression density] 0.5 to 3.0 g / cm ³ , preferably 1.0
２2.0 g / cm ³ . [True specific gravity] It is desirably 3.0 to 6.0 g / cm ³ , and more preferably 3.5 to 4.3 g / cm ³ . If the compression density and tap density relative to the true specific gravity are high, the powder tends to be compacted in the coating film when calendering is performed during the tape forming process, which is advantageous in improving the tape surface smoothness. I do. [Crystal Grain Size] (Crystallite) It is 10 to 200 angstroms, preferably 50 to 150 angstroms.

The particle size is, as required, an average major axis length of 0.01 to 0.5 μm and an average minor axis length of 0.01 to 0.05.
μm, an average axis ratio of 1 to 30 is desirable, and a specific surface area is 10 to 10.
300 m ² / g is desirable, and the crystal grain size is 10 to 200 A
However, in such fine particles, the length of the shortest axis (shortest axis length) affects the surface smoothness of the tape, and the shortest axis length improves the surface smoothness. The shortest axis length is reflected in the crystal grain size and the specific surface area.

Further, the surface treatment state and pH of the powder also affect the dispersibility at the time of forming a paint, and thus affect the surface smoothness. These preferred ranges are as follows, and it is desirable to adjust to these ranges. [Stearic acid adsorption amount] 0.1 to 3.0 mg / m ² . [Resin adsorption amount] 0.5 to 4.0 mg / m ² . [PH] Powder pH is 6-11, preferably 8-10,
More preferably, it is 8.0 to 9.5. By adjusting the pH, the dispersibility at the time of coating is improved, which effectively works to improve the surface smoothness.

The powder for the lower layer according to the present invention can be obtained by a usual method for producing iron oxyhydroxide powder. For example, a suspension containing ferrous hydroxide colloid obtained by adding an equivalent amount or more of an aqueous alkali hydroxide solution to an aqueous ferrous salt solution is subjected to an oxidation reaction by passing an oxygen-containing gas at a pH of 11 or more and a temperature of 80 ° C. or less. And a method of producing by drying and then adjusting the humidity. Alternatively, an oxygen-containing gas is passed through a suspension obtained by reacting an aqueous solution of ferrous salt with an aqueous solution of alkali carbonate to carry out an oxidation reaction, and the drying is adjusted. Examples of the method include a method of producing by wetting. The iron oxyhydroxide powder obtained by such a method does not have a processing step at a high temperature as compared with the case of producing needle-like iron oxide (hematite) powder, so that there is no problem of interparticle sintering.

FIG. 1 shows that the major axis length = 0.30 μm and Al = 2.
5 is a TEM (transmission electron microscope) photograph of the iron oxyhydroxide powder according to the present invention having a specific surface area (BET) of 8% by weight and a specific surface area (BET) of 65 m ² / g. As shown in FIG. 1, each particle has a branch, but the branching direction is deviated in a direction parallel to the paper surface. This can be seen from the fact that even if the branching has three or more branches, the branching angle looks almost constant. This is because if there are many components in the direction perpendicular to the paper surface, the branching angle should look sharper. Thus, even if each particle has multiple branches,
The fact that the branching direction of each particle is biased in a certain two-dimensional direction contributes to the surface smoothness when the powder composed of the particles is used as the lower layer powder. This is because, when coated, there are few branching components in the direction perpendicular to the support surface. And, having the branches contributes to the improvement of the tape strength because they are entangled with each other.

In particular, when the major axis length is 0.5 μm or less, when this is dispersed in a resin binder and applied to a support, very good surface smoothness is exhibited. As can be seen in FIG.
Such fine needle-like particles of iron oxyhydroxide are characterized by a very short minor axis length and a high needle-like ratio, so that they are well oriented in the longitudinal direction of the tape during application (the branching direction is also in this direction). Orientation), and the tape strength is improved in addition to the surface smoothness.

Further, when an appropriate amount of Al is contained in the iron oxyhydroxide, heat resistance and storage stability can be increased. When the content of Al is 0.1 to 30% by weight, the iron oxyhydroxide powder can be stably present without being deteriorated even when the temperature is raised in the drying step in forming the tape. Al content is 0.
If the amount is less than 1% by weight, the effect of Al content is insufficient. If the Al content is more than 30% by weight, the specific surface area of the powder becomes large and the dispersibility becomes poor. Here, the content of Al refers to the content of the Al element, not the amount of the compound when Al is contained as a compound. Al may be contained as a solid solution in the iron oxyhydroxide or may be adhered to the surface of the iron oxyhydroxide.

To make the iron oxyhydroxide contain Al, water-soluble salts such as Al ₂ (SO ₄ ) ₃ , Al (NO ₃ ) ₃ , and AlCl _3, and NaAlO ₂ (sodium aluminate) Compounds such as water-soluble aluminate can be used. In order to “adhere” Al onto the surface of iron oxyhydroxide particles using these Al compounds, for example, these Al compounds are dissolved in an aqueous alkali solution, and the iron oxyhydroxide is dispersed in this solution. After that, it can be carried out by blowing carbon dioxide gas or adding an acid to neutralize, and Al is deposited on the particle surface as crystalline or amorphous Al ₂ O ₃ .nH ₂ O (hydrous aluminum oxide). Is done. On the other hand, to “dissolve” Al in iron oxyhydroxide particles, an aqueous solution of a ferrous salt such as FeSO ₄ or FeCl ₂ is neutralized with a neutralizing agent such as NaOH, Na ₂ CO ₃ , and NH ₄ OH. And then oxidized by air or the like to make α-FeOOH, γ-Fe
The above water-soluble Al
A salt or an aluminate may be added.

In the powder according to the present invention, the particle surface property may be controlled by using another element such as a Si compound. When Si is contained, the content is in the range of 0.1 to 30% by weight. When Al and Si are contained, the total amount of both is preferably in the range of 0.1 to 30% by weight. Here, the content of Si refers to the content of the Si element, not the amount of the Si compound, even when Si is contained as a compound.

It has been found that the decomposition temperature of the iron oxyhydroxide when heated in the atmosphere changes depending on the Al content in the iron oxyhydroxide. FIG. 2 shows the decomposition start temperature and the decomposition end temperature when the Al content (% by weight) in the iron oxyhydroxide was changed. These decomposition temperatures are in accordance with JIS K 712.
It was measured by a differential thermal analyzer according to 0. The curves connecting the stars shown in FIG. 2 are deduced from plots of the measured values. As can be seen from this curve, both the decomposition onset temperature of iron oxyhydroxide and the decomposition onset temperature are Al
It can be seen that the content increases as the content increases. The representative values of each curve are as follows.

[0026]

The water contained in the iron oxyhydroxide affects the properties of the lower layer. The amount of water released at 100 ° C. must be 2% by weight or less, and preferably the amount of water released at 100 ° C. is 1.5% by weight or less. If the amount of water released at 100 ° C. is more than 2% by weight, the dispersion in the binder resin becomes insufficient, and it becomes difficult to form a tape even when applied. This moisture content can be measured using the principle of moisture measurement by the Karl Fischer method. One example is shown below.

[0028]

In the case of forming a lower layer using the iron oxyhydroxide powder according to the present invention in a magnetic recording medium having a multilayer structure,
The upper magnetic layer is preferably formed using acicular metal powder having the following composition.

[Component composition of upper metal powder] Co: 5 to 50 at.% Al: 0.1 to 30 at.% Rare earth element (including Y): 0.1 to 10 at.%, Group 1a element of the periodic table : 0.05% by weight or less, Group 2a element of the periodic table: 0.1% by weight or less (including 0% by weight) in Fe, and the amount of H ₂ O released at 100 ° C. is 2%.
_3. The amount of H ₂ O released at below 300 wt% or at 300 ° C.
Needle-like ferromagnetic metal powder of 0% by weight or less.

Here, examples of Group 1a elements of the periodic table include Li, Na, K and the like. Even when these are contained in combination, the total amount is set to 0.05% by weight or less.
Examples of Group 2a elements of the periodic table include Mg, Ca, S
r, Ba and the like, and when these are contained in combination, the total amount is set to 0.1% by weight or less. The rare earth elements include Y, La, Ce, Pr, Nd, Sm, T
b, Dy, Gd, etc., and when these are contained in combination, the total amount is 0.1 to 10 at.%. Even when these elements are contained as a compound, the content of the element in the compound is not the amount of the compound.

Preferred shapes and physical properties that the metal powder should have are as follows. [Long axis length]: 0.01 to 0.4 μm [Specific surface area]: 30 to 70 m ² / g by BET method [Crystal grain size]: 50 to 250 Å [Coercive force Hc]: 1200 to 3000 (Oe) [ Saturation magnetic flux density [sigma] _S ]: 100 to 200 (emu / g)

In order to produce this metal powder, a predetermined amount of Al is added to iron oxyhydroxide or iron oxide containing Co.
A method of reducing this by heating is preferred. Compound powders mainly containing iron oxyhydroxide or iron oxide to be subjected to the heat reduction include α-FeOOH, γ-FeOOH,
-Fe ₂ O ₃ , γ-Fe ₂ O ₃ , Fe ₃ O ₄ and those corresponding to the intermediate type thereof, and Ni, Cr, Mn, Z
Those containing a metal component such as n are preferred, and those having good needle-like properties are preferred. At this time, Al compounds that can be used to contain Al include Al ₂
Examples thereof include water-soluble salts such as (SO ₄ ) ₃ , Al (NO ₃ ) ₃ , and AlCl ₃ , and water-soluble aluminates such as NaAlO ₂ . In order to apply these Al compounds to the particle surfaces of the object to be reduced, usually, these Al compounds are dissolved in an aqueous alkaline solution, and after the powder of the object to be reduced is dispersed in the solution, carbon dioxide gas is blown. This is performed by adding an acid to neutralize the crystalline or amorphous Al ₂
O ₃ .nH ₂ O (hydrous aluminum hydroxide) is applied to the particle surface. Alternatively, a method in which Al is dissolved in the particles of the substance to be reduced may be used.

Α-FeOOH containing Co, γ-FeOO
To form a solid solution of Al in H, an aqueous solution mainly containing a ferrous salt such as FeSO ₄ or FeCl ₂ is added to NaOH, Na ₂ C
After neutralizing with a neutralizing agent such as O ₃ or NH ₄ OH and then oxidizing with air or the like to generate α-FeOOH, γ-FeOOH, etc., the above water-soluble Al salt or aluminate is added. I just need. Further, in order to dissolve Al in α-Fe ₂ O ₃ containing Co, an aqueous solution of a ferric salt such as Fe ₂ (SO ₄ ) ₃ and FeCl ₃ and a neutralizing agent such as NaOH and KOH are used. The water-soluble Al salt or aluminate may be added to a reaction system for synthesizing α-Fe ₂ O ₃ by a thermal synthesis method.

The Al-containing iron oxyhydroxide or iron oxide containing Co thus obtained is heated to convert Al to Al ₂
It is preferable to fix it as O _{3 and} use this as a raw material in the step of containing Y (including rare earth elements).
At this time, the iron oxyhydroxide is transformed into iron oxide by a dehydration reaction. A method in which raw material particles are dispersed in a liquid containing Y, and an alkali is added to precipitate in the form of a hydroxide;
There is a method of dispersing raw material particles in an element compound-containing liquid and evaporating water.

The iron oxide powder containing predetermined amounts of Co, Al and Y (including a rare earth element) by the above-mentioned various methods is
By heating in a reducing atmosphere, the metal magnetic powder is reduced and becomes a metallic magnetic powder containing Co, Al and Y (including a rare earth element) containing iron as a main component.

The content of Y (including rare earth elements) in the metal magnetic powder is 0.1 to 10 atomic%, preferably 0.2 to 5 atomic%.
Is good. If the content is less than 0.1 atomic%, the effect of Y (rare earth element) is small and sintering becomes easy.
The amount of (rare earth element) oxide increases and the saturation magnetization decreases, which makes the metal magnetic powder unsuitable for the upper layer.

The Al content of the metal magnetic powder is 0.1 to 30.
Atomic%, preferably 1 to 20 atomic%.
If it is less than 0.1 atomic%, sintering becomes easy and 30 atomic%
If it exceeds, the saturation magnetization becomes small.

In the metal magnetic powder, the periodic table 1a
Group element is 0.05% by weight or less and group 2a element is
In order to reduce the content by weight to less than 1% by weight, it is necessary to use, as raw materials, those which do not contain elements of Groups 1a and 2a of the Periodic Table or those whose content is as low as possible. It is preferable to perform sufficient washing at the stage of each compound of the powder to remove the powder. In the case of cleaning, the above elements are segregated on the particle surface as the process proceeds, so that the cleaning efficiency is improved. Further, if warm water or an acid is added to the washing water to reduce the pH, the washing water can be more efficiently removed.

When the Group 1a element exceeds 0.05% by weight, the compatibility with the resin is deteriorated when the tape is formed, so that the element cannot be dispersed. Further, since this element is soluble, when the tape is held for a certain period of time, it precipitates on the tape surface to become a crystalline compound, and this compound causes an increase in dropout and the like, and lowers the storage stability of the tape. On the other hand, if the Group 2a element exceeds 0.1% by weight, the compatibility with the resin is deteriorated and the coating film strength is lowered. If the Group 2a element is extremely large, the tape storage stability is deteriorated like the Group 1a element.

The amount of water contained in the metallic magnetic powder is detected (released) at 100 ° C. at 2.0% by weight or less, preferably 1.5% by weight or less, and is detected (released) at 300 ° C. Is 4.0% by weight, preferably 3.0% by weight or less. The viscosity of the paint changes depending on the amount of water held by the metal magnetic powder, and the amount of binder adsorbed also changes. However, if the amount of water detected at 100 ° C. exceeds 2.0% by weight, or
If the amount of water detected at 0 ° C. exceeds 4.0% by weight, it is difficult to form a tape due to insufficient dispersion when coating the lower layer with multiple layers.

The particle size of the metal magnetic powder is 0.01 to 0.4.
μm is appropriate, and preferably 0.4 to 0.2 μm.
If it is less than 0.01 μm, the magnetic powder becomes superparamagnetic and the electromagnetic conversion characteristics are significantly reduced. If it exceeds 0.4 μm, the magnetic powder becomes multi-domain and the electromagnetic conversion characteristics deteriorate.

The specific surface area (BET) of the metal magnetic powder is 30 to
70m ² / g is suitable, preferably of 40 to 60 ² / g is good. If it is less than 30 m ² / g, the compatibility with the resin at the time of forming into a tape becomes poor, and the electromagnetic conversion characteristics deteriorate. 70m ² / g
If the ratio exceeds the above range, poor dispersion occurs at the time of forming into a tape, and the electromagnetic conversion characteristics also deteriorate.

The crystallite of the metal magnetic powder is suitably from 50 to 250 angstroms, and preferably from 100 to 200 angstroms. If the thickness is less than 50 angstroms, the magnetic powder becomes superparamagnetic and the electromagnetic conversion characteristics are significantly reduced. If it exceeds 250 angstroms, noise increases and electromagnetic conversion characteristics deteriorate.

The higher the coercive force Hc, the better the magnetic properties of the metal magnetic powder are for high-density recording.
It is controlled at 200 to 3000 (Oe), preferably 1600 to 2600 (Oe). The higher the saturation magnetic flux density σ _s, the higher the output, but it is preferably about 120 to 180 emu / g in consideration of oxidation resistance and noise.

As the support on which the lower and upper layers are coated to form a multilayered magnetic recording medium, polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins, cellulose triacetate, polycarbonate, polyamide, polyimide, Known films such as polyamide imide, polysulfone aramid, and aromatic polyamide can be used.

[0047]

【Example】

(1) As shown in Table 1, examples of lower layer powder according to the present invention (lower layer examples 1 to 20) and lower layer comparative examples 1 to 8
(2) As shown in Tables 2 and 3, Examples of a magnetic recording medium constituted by applying an upper layer using metal magnetic powder to a lower layer using the lower layer powder according to the present invention [medium examples 1 to 15] Hereinafter, the medium comparative examples 1 to 13 will be described.

First, measurement of characteristic values shown in each embodiment will be described.

The average major axis length (indicated by I in the table), the average minor axis length (indicated by d) and the axial ratio (indicated by I / d) are 1
08000 times (in case of lower layer powder) or 174000
The average value of 100 particles measured from an electron micrograph of × (in the case of the metal magnetic powder in the upper layer) was shown. The crystal grain size, that is, the crystallite (the same Dx) is calculated by obtaining the half-value width of the peak corresponding to the (110) plane from the profile obtained by using an X-ray diffraction apparatus, and substituting this into Scherrer's equation. did.

The specific surface area (BET) was measured by the BET method. Stearic acid adsorption amount (same STA or St. adsorption amount)
Was obtained by dispersing a sample powder in a 2% MEK solution of stearic acid, then sinking the sample powder by a centrifugal separator, and calculating the concentration of the supernatant to calculate the amount of adsorption per specific surface area. The resin adsorption amount (the same resin) was calculated in the same manner as the stearic acid adsorption amount using a 2% MIBK solution of a polyurethane resin.

The powder pH was measured according to JIS K5101. The true specific gravity was measured by a liquid immersion method using toluene as a solvent. Compressed density (CD) is 80 kgf / cm
^This is the density when compressed by ² . Tap density (TA
P) was measured according to JIS K5101.

The water content of the powder was determined from the weight change at 100 ° C. (or 300 ° C.) by the Karl Fischer method. In addition, the decomposition start temperature and end temperature were determined from the differential heat data. The change in viscosity due to the amount of water was determined by measuring the viscosity of the paint when dispersed in the paint using an E-type viscometer.

The surface smoothness was measured by Kosaka Laboratory Co., Ltd.
The evaluation was performed by measuring the Ra (roughness) of the surface of the underlayer of the tape using a dimensional fine shape measuring device (ET-30HK).

In each table, Hc: coercive force (Oe), σs: saturation magnetic flux density of metal magnetic powder (emu / g), σr: residual magnetic flux density of metal magnetic powder (emu / g), Br: tape Magnetic flux density (Gauss), Bm: Saturated magnetic flux density of the tape (Gauss), σr / σs and Br / Bm: Squareness ratio Δσs and ΔBm: After standing at 60 ° C. in an atmosphere of 90 RH (relative humidity) for one week The reduction rate (%) of σs and Bm, the presence or absence of precipitates after the weathering test: the presence or absence of precipitates when the tape surface was left under an atmosphere of 90 RH at 60 ° C. for one week and observed with a microscope. The measurement of the electromagnetic conversion characteristics was performed using a Hi8 deck.

(1) Examples of lower layer powder [Lower layer examples 1-2]
0] and lower layer comparative examples 1 to 8

[Lower layer example 1] A paint having the following composition is prepared. 100 parts by weight of iron oxyhydroxide (in this example, major axis length = 0.15 μm, water content at 100 ° C. =
Polyurethane resin 20 parts by weight Methyl ethyl ketone 165 parts by weight Cyclohexanone 65 parts by weight Toluene 165 parts by weight Stearic acid 1 part by weight acetylacetone 1 part by weight A paint of the above composition obtained by dispersing for 1 hour by a centrifugal ball mill was prepared from polyethylene terephthalate. The target thickness is about 3 μm on the base film using an applicator.
To form a nonmagnetic lower layer. The various characteristic values of the iron oxyhydroxide powder used and the properties of the obtained lower layer are shown in Table 1 (the following examples and comparative examples are also shown in Table 1).

[Lower Layer Example 2] Iron oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of Example 1, was replaced with a major axis length =
0.15 μm, Al = 0.2% by weight Iron oxyhydroxide was applied, and the other conditions were the same as those of the lower layer example 1 to form a lower layer.

[Lower Layer Example 3] The iron oxide oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was prepared.
0.15 μm, Al = 1.0% by weight Iron oxyhydroxide was applied, and the other conditions were the same as the lower layer example 1 to form a lower layer.

[Lower Layer Example 4] The iron oxide oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was prepared.
0.15 μm, Al = 2.5% by weight Iron oxyhydroxide was applied, and the other conditions were the same as the lower layer example 1 to form a lower layer.

[Lower Layer Example 5] The iron oxide oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was prepared.
0.15 μm, Al = 5.0% by weight Iron oxyhydroxide to be applied was used, and the other conditions were the same as the lower layer example 1 to form a lower layer.

[Lower Layer Example 6] Iron oxide oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was prepared using the following method.
0.15 μm, Al = 30.0% by weight Iron oxyhydroxide was applied, and the other conditions were the same as those of the lower layer example 1 to form a lower layer.

[Lower Layer Example 7] The iron oxide oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was prepared.
0.15 μm, Al = 1.0% by weight solid iron oxide oxyhydroxide was used, and other conditions were the same as the lower layer example 1 to form a lower layer.

[Lower layer example 8] Iron oxide oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was prepared using the following formula.
0.15 μm, Al = 2.5% by weight of solid solution was changed to iron oxyhydroxide, and the other conditions were the same as the lower layer example 1 to form a lower layer.

[Lower Layer Example 9] The iron oxide oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was prepared as follows.
The lower layer was changed to 0.15 μm, Al = 5.0 wt% solid solution iron oxyhydroxide, and the other conditions were the same as the lower layer example 1.

[Lower Layer Example 10] The iron oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was replaced by a solid solution of oxyhydroxide having a major axis length of 0.15 μm and Al = 10.0% by weight. Instead of iron hydroxide, the other conditions were the same as in the lower layer example 1 to form a lower layer.

[Lower Layer Example 11] The iron oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was replaced by a solid solution of oxyhydroxide having a major axis length of 0.15 μm and Al = 20.0% by weight. Instead of iron hydroxide, the other conditions were the same as in the lower layer example 1 to form a lower layer.

[Lower layer example 12] Iron oxide oxyhydroxide having a major axis length of 0.15 µm constituting the coating material of the lower layer example 1 was changed to iron oxyhydroxide having a major axis length of 0.10 µm. A lower layer was formed in the same manner as in Lower Layer Example 1.

[Lower Layer Example 13] The iron oxide oxyhydroxide having a major axis length of 0.15 μm constituting the paint of the lower layer example 1 was changed to iron oxyhydroxide having a major axis length of 0.30 μm. A lower layer was formed in the same manner as in Lower Layer Example 1.

[Lower Layer Example 14] The iron oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was prepared by coating the oxyhydroxide having a major axis length of 0.05 μm and Al = 5.0% by weight. Instead of iron hydroxide, the other conditions were the same as in the lower layer example 1 to form a lower layer.

[Lower Layer Example 15] The iron oxide oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was replaced with an oxyhydroxide having a major axis length of 0.10 μm and Al = 5.0% by weight. Instead of iron hydroxide, the other conditions were the same as in the lower layer example 1 to form a lower layer.

[Lower Layer Example 16] The iron oxide oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was coated with oxyhydroxide having a major axis length of 0.30 μm and Al = 5.0% by weight. Instead of iron hydroxide, the other conditions were the same as in the lower layer example 1 to form a lower layer.

[Lower Layer Example 17] Iron oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was replaced with a solid solution of oxyhydroxide having a major axis length of 0.05 μm and Al = 5.0 wt%. Instead of iron hydroxide, the other conditions were the same as in the lower layer example 1 to form a lower layer.

[Lower Layer Example 18] The iron oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was replaced by a solid solution of oxyhydroxide having a major axis length of 0.10 μm and Al = 5.0% by weight. Instead of iron hydroxide, the other conditions were the same as in the lower layer example 1 to form a lower layer.

[Lower Layer Example 19] Iron oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was replaced with an oxyhydroxide having a major axis length of 0.30 μm and Al = 5.0% by weight. The lower layer was changed to iron hydroxide and the water content was changed at three levels: (A) 0.5% by weight, (B) 1.0% by weight and (C) 2.0%.

[Lower Layer Example 20] The iron oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was replaced by a solid solution of oxyhydroxide having a major axis length of 0.50 μm and Al = 5.0% by weight. Instead of iron hydroxide, the other conditions were the same as in the lower layer example 1 to form a lower layer.

[Lower Layer Comparative Example 1] Iron oxide oxyhydroxide having a major axis length of 0.15 μm constituting the paint of the lower layer example 1 was changed to α-Fe ₂ O ₃ having a major axis length of 0.15 μm. Are substantially the same as those of the lower layer example 1 to form a lower layer.

[Lower Layer Comparative Example 2] The iron oxide oxyhydroxide having a major axis length of 0.15 μm constituting the paint of the lower layer example 1 was changed to titanium oxide having an average diameter of 0.035 μm. The lower layer was made substantially the same as that of No. 1.

[Lower layer comparative example 3] Iron oxide oxyhydroxide having a major axis length of 0.15 µm, which constitutes the coating material of the lower layer example 1, was coated with a major axis length of 0.15 µm and Al = 35.0% by weight. The lower layer was changed to iron oxyhydroxide and the other conditions were the same as those of the lower layer example 1.

[Lower Layer Comparative Example 4] The iron oxide oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was dissolved in a solid solution with a major axis length of 0.15 μm and Al of 35.0% by weight. The lower layer was changed to iron oxyhydroxide and the other conditions were the same as those of the lower layer example 1.

[Lower layer comparative example 5] Iron oxide oxyhydroxide having a major axis length of 0.15 µm constituting the coating material of the lower layer example 1 was changed to iron oxyhydroxide having a major axis length of 0.005 µm. Was the same as the lower layer example 1 to form a lower layer.

[Lower layer comparative example 6] The iron oxide oxyhydroxide having a major axis length of 0.15 µm constituting the coating material of the lower layer example 1 was changed to iron oxyhydroxide having a major axis length of 0.60 µm. Was the same as the lower layer example 1 to form a lower layer.

[Lower layer comparative example 7] Iron oxide oxyhydroxide having a major axis length of 0.15 µm, which constitutes the coating material of the lower layer example 1, was coated with a major axis length of 0.60 µm and Al = 5.0% by weight. The lower layer was changed to iron oxyhydroxide and the other conditions were the same as those of the lower layer example 1.

[Lower Layer Comparative Example 8] The iron oxyhydroxide having a major axis length of 0.15 μm, which constitutes the coating material of the lower layer example 1, was dissolved in a solid solution of a major axis length of 0.60 μm and Al = 5.0% by weight. The lower layer was changed to iron oxyhydroxide and the other conditions were the same as those of the lower layer example 1.

[0084]

[Table 1]

As can be seen from the results in Table 1, the underlayer using the iron oxyhydroxide powder according to the present invention has a small roughness, excellent surface smoothness and sufficient strength as compared with those of the comparative example. You can see that.

(2) Embodiment of recording medium [medium examples 1 to 17]
And medium comparative examples 1 to 13

[Medium Example 1] A lower layer paint having the following composition is prepared. 100 parts by weight of iron oxyhydroxide (in this example, major axis length = 0.15 μm, water content at 100 ° C. =
1.0% by weight) Polyurethane resin 20 parts by weight Methyl ethyl ketone 165 parts by weight Cyclohexanone 65 parts by weight Toluene 165 parts by weight Stearic acid 1 part by weight acetylacetone 1 part by weight A coating material of the above composition obtained by dispersing for 1 hour by a centrifugal ball mill is polyethylene terephthalate. Was applied using a applicator on a base film made of to form a lower layer. Table 2 shows various characteristic values of the iron oxyhydroxide powder used and properties of the obtained lower layer (the following examples and comparative examples are also shown in Table 2).

An upper layer paint having the following composition is prepared. 100 parts by weight of metal magnetic powder (In this example, in metal Fe, Co: 30 at.%, Al: 1
0 at.%, Y: 4 at.%, Na: 0.002 wt%, Ca: 0.
Polyurethane resin 30 parts by weight Methyl ethyl ketone 190 parts by weight Cyclohexanone 80 parts by weight Toluene 110 parts by weight Stearin butyl 1 part by weight acetylacetone 1 part by weight α-alumina 3 parts by weight Carbon black 2 parts by weight Disperse by a centrifugal ball mill for 1 hour. The upper layer paint thus obtained was applied on the lower layer using an applicator to form a sheet-like sample, which was further calendered and slit to a width of 8 mm to obtain a magnetic tape.
Various characteristic values of the metal magnetic powder used and properties of the obtained magnetic tape are shown in Tables 2 and 3 (the following examples and comparative examples are also shown in Tables 2 and 3).

[Medium Example 2] Iron oxyhydroxide having a major axis of 0.15 μm constituting the lower layer of Medium Example 1 was replaced with 0.30 μm of major axis.
m was changed to iron oxyhydroxide, and the other conditions were the same as in Medium Example 1 to obtain a magnetic tape.

[Medium Example 3] The amount of Co, the amount of Y, and the major axis length of the metal magnetic powder constituting the upper layer of Medium Example 1 were expressed as Co: 10a
A magnetic tape was obtained in the same manner as in Example 1 except that the metal magnetic powder was changed to t.%, Y: 2 at.%, major axis length: 0.095 μm.

[Medium Example 4] Iron oxyhydroxide having a major axis of 0.15 μm constituting the lower layer of Medium Example 1 was replaced with 0.30 μm of major axis.
m, Al: 5% by weight of iron oxyhydroxide to be adhered, and the other conditions were the same as in Medium Example 1 to obtain a magnetic tape.

[Medium Example 5] Iron oxyhydroxide having a major axis of 0.15 μm constituting the lower layer of Medium Example 1 was replaced with 0.30 μm of major axis.
m, Al: A magnetic tape was obtained in the same manner as in Example 1, except that iron oxyhydroxide was used as a solid solution in an amount of 5% by weight.

[Medium Example 6] The Co amount, Al amount, Y amount, and long axis length of the metal magnetic powder constituting the upper layer of Medium Example 5 are represented by Co:
30 at.%, Al: 8 at.%, Y: 3 at.%, Major axis length: 0.0
A magnetic tape was obtained in the same manner as in the medium example 5 except that the magnetic tape was changed to 8 μm.

[Medium Example 7] Iron oxyhydroxide having a major axis of 0.15 μm constituting the lower layer of Medium Example 1 was replaced with 0.10 μm of major axis.
m, Al: A magnetic tape was obtained in the same manner as in Example 1, except that iron oxyhydroxide was used as a solid solution in an amount of 5% by weight.

[Medium Example 8] A magnetic tape was obtained in the same manner as in Medium Example 5 except that the amount of Y in the metal magnetic powder constituting the upper layer of Medium Example 5 was changed to La: 4 at.%.

[Medium Example 9] The Co amount, Al amount, Y amount and major axis length of the metal magnetic powder constituting the upper layer of Medium Example 5 are represented by Co:
10 at.%, Al: 10 at.%, Y: 4 at.%, Major axis length: 0.1
The magnetic tape was obtained in the same manner as in Example 5 except that the magnetic tape was changed to 08 μm.

[Medium Example 10] The amount of Co, the amount of Al, the amount of Y, and the major axis length of the metal magnetic powder constituting the upper layer of Medium Example 5 were expressed as C.
o: 20 at.%, Al: 10 at.%, Y: 4 at.%, major axis length:
A magnetic tape was obtained in the same manner as in Medium Example 5 except that the tape was changed to 0.08 μm.

[Medium Example 11] The amount of Co, the amount of Al, the amount of Y, and the major axis length of the metal magnetic powder constituting the upper layer of Medium Example 5 were expressed as C.
o: 40 at.%, Al: 10 at.%, Y: 4 at.%, major axis length:
A magnetic tape was obtained in the same manner as in Medium Example 5 except that the tape was changed to 0.08 μm.

[Medium Example 12] The amount of Co, the amount of Al, the amount of Y, and the length of the major axis of the magnetic metal powder constituting the upper layer of Medium Example 5 were expressed as C.
o: 50 at.%, Al: 10 at.%, Y: 4 at.%, major axis length:
A magnetic tape was obtained in the same manner as in Medium Example 5 except that the tape was changed to 0.08 μm.

[Medium Example 13] The Co amount, Al amount, Y amount, Na amount, Ca amount and major axis length of the metal magnetic powder constituting the upper layer of Medium Example 5 were Co: 30 at.%, Al: 10 at.%. , Y: 4
at.%, Na: 0.006% by weight, Ca: 0.12% by weight,
A magnetic tape was obtained in the same manner as in Medium Example 5 except that the major axis length was changed to 0.08 μm.

[Medium Example 14] The iron oxyhydroxide constituting the lower layer of Medium Example 1 was changed to a medium having a major axis length of 0.30 μm and a content of 2.5% by weight of Si. A magnetic tape was obtained in the same manner.

[Medium Example 15] Iron oxyhydroxide constituting the lower layer of Medium Example 1 was changed to one having a major axis length of 0.30 μm, a solid solution of Al: 5% by weight, and a content of Si: 2.5% by weight. A magnetic tape was obtained under the same conditions as in Example 1 except for the above conditions.

Medium Comparative Example 1 Iron oxyhydroxide constituting the lower layer of Medium Example 1 was replaced with α-Fe ₂ having a major axis of 0.15 μm.
Changed to O _3, other conditions to obtain a magnetic tape in the substantially identical to that of the medium Example 1.

[Medium Comparative Example 2] The iron oxyhydroxide constituting the lower layer of Medium Example 1 was changed to titanium oxide having an average particle diameter of 0.035 μm, and other conditions were substantially the same as those of Medium Example 1. A magnetic tape was obtained in the same manner.

[Medium Comparative Example 3] The iron oxyhydroxide constituting the lower layer of Medium Example 1 was prepared by changing the major axis to 0.30 μm and the Al to 3
A magnetic tape was obtained in the same manner as in Example 1 except that the solid solution was changed to a solid solution of 5% by weight.

[Medium Comparative Example 4] The amount of Co, the amount of Al, the amount of Y, and the major axis length of the metal magnetic powder constituting the upper layer of Medium Example 5 were expressed as C
o: 3 at.%, Al: 10 at.%, Y: 4 at.%, major axis length 0.
The magnetic tape was obtained in the same manner as in Example 5 except that the magnetic tape was changed to 08 μm.

[Medium Comparative Example 5] The amount of Co, the amount of Al, the amount of Y, and the length of the major axis of the metallic magnetic powder constituting the upper layer of Medium Example 5 were expressed as C.
o: 55 at.%, Al: 10 at.%, Y: 4 at.%, major axis length was changed to 0.08 μm, and the other conditions were the same as in Medium Example 5 to obtain a magnetic tape.

[Medium Comparative Example 6] The amount of Co, the amount of Al, the amount of Y, and the major axis length of the metal magnetic powder constituting the upper layer of Medium Example 5 were represented by C
o: 30 at.%, Al: 0 at.%, Y: 4 at.%, major axis length: 0.
The magnetic tape was obtained in the same manner as in Example 5 except that the magnetic tape was changed to 08 μm.

[Medium Comparative Example 7] The amount of Co, the amount of Al, the amount of Y, and the major axis length of the metal magnetic powder constituting the upper layer of Medium Example 5 were expressed as C.
o: 30 at.%, Al: 35 at.%, Y: 4 at.%, major axis length: 0.08 μm, and the other conditions were the same as in Medium Example 5 to obtain a magnetic tape.

[Medium Comparative Example 8] The amount of Co, the amount of Al, the amount of Y, and the length of the major axis of the metal magnetic powder constituting the upper layer of Medium Example 5 were represented by C
o: 30 at.%, Al: 10 at.%, Y: 0 at.%, major axis length: 0.08 μm, and the other conditions were the same as in Medium Example 5 to obtain a magnetic tape.

[Medium Comparative Example 9] The amount of Co, the amount of Al, the amount of Y, and the length of the major axis of the metallic magnetic powder constituting the upper layer of Medium Example 5 were expressed as C.
o: 30 at.%, Al: 10 at.%, Y: 15 at.%, major axis length was changed to 0.08 μm, and the other conditions were the same as in Medium Example 5 to obtain a magnetic tape.

[Medium Comparative Example 10] The amount of Co, the amount of Al, the amount of Y, the amount of Na, and the amount of Ca in the metallic magnetic powder constituting the upper layer of Medium Example 5
Co and 30 at.%, Al: 10 at.%,
Y: 4 at.%, Na: 0.16% by weight, Ca: 0.12% by weight, major axis length: 0.08 μm, and the other conditions were the same as in Medium Example 5 to obtain a magnetic tape. .

[Medium Comparative Example 11] The iron oxyhydroxide constituting the lower layer of Medium Example 1 was prepared with a major axis length of 0.30 μm and Al:
A magnetic tape was obtained in the same manner as in Example 1 except that the content was changed to 5% by weight solid solution and 35% by weight of Si.

[Medium Comparative Example 12] The water content of the metal magnetic powder constituting the upper layer of the medium comparative example 11 was changed to 3.5% by weight at 100 ° C. and 5.5% by weight at 300 ° C. The magnetic tape was obtained under the same conditions as in the medium comparative example 11.

[Medium Comparative Example 13] The water content of the iron oxyhydroxide constituting the lower layer of the medium comparative example 11 was adjusted to 100.degree.
A magnetic tape was obtained in the same manner as in Comparative Example 11 except that the weight was changed to 5% by weight.

[0116]

[Table 2]

[0117]

[Table 3]

From Tables 2 and 3, it can be seen that a magnetic recording medium having a multilayer structure using the lower layer powder of iron oxyhydroxide according to the present invention has excellent strength, surface roughness, magnetic conversion characteristics, and weather resistance. It turns out that it becomes. In addition, as the metal magnetic powder in the upper layer, the 1a group and 2a
Those containing a large amount of group elements Na and Ca are difficult to disperse, and the tape durability is low even when dispersed, and those stored for 1 week at 60 ° C. and 90 RH have poor storage stability when crystals are precipitated by observing the tape surface. It became. Further, it can be seen that when the amounts of Co, Y and Al coexist in the metal magnetic powder, the magnetic properties are further improved, and that the use of the lower layer according to the present invention makes it possible to advantageously bring out these magnetic properties.

[0119]

As described above, the lower layer powder of the present invention improves the quality of a coating type magnetic recording medium having a multilayer structure, specifically, improves the surface smoothness, strength, magnetic properties, weather resistance, etc. Therefore, a high-density magnetic recording medium having good electromagnetic conversion characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is an electron micrograph showing the shape (branched state) of individual particles of a lower layer powder composed of acicular iron oxyhydroxide according to the present invention.

FIG. 2 is a diagram showing the relationship between the Al content in iron oxyhydroxide and the decomposition temperature of iron oxyhydroxide.

Continuing from the front page (72) Inventor Shinichi Konno 1-8-2 Marunouchi, Chiyoda-ku, Tokyo Dowa Mining Co., Ltd. (72) Inventor Yoshifumi Horikawa 1-8-2 Marunouchi, Chiyoda-ku, Tokyo Dowa Mining Co., Ltd. In-company (72) Inventor Yasuhiko Awaihara 1-8-2 Marunouchi, Chiyoda-ku, Tokyo Dowa Mining Co., Ltd.

Claims (7)

[Claims] [Claim 1] amount of H ₂ O released at will and 100 ° C. acicular particles having an average major axis length 0.01~0.5μm consists of 2 wt% or less of the iron oxyhydroxide powder coating type magnetic recording Lower layer powder for media. 2. Oxygen consisting of needle-like particles having an average major axis length of 0.01 to 0.5 μm having a two-dimensional branching direction and having an amount of H ₂ O released at 100 ° C. of 2% by weight or less. Lower layer powder for coating type magnetic recording media consisting of iron hydroxide powder. 3. The composition of needle-like particles having an average major axis length of 0.01 to 0.5 μm, containing 0.1 to 30% by weight of Al and releasing ₂ % by weight of H ₂ O at 100 ° C. Lower layer powder for a coating type magnetic recording medium comprising the following iron oxyhydroxide powder. 4. It is composed of needle-shaped particles having an average major axis length of 0.01 to 0.5 μm, contains 0.1 to 30% by weight of Si, and releases ₂ % by weight of H ₂ O at 100 ° C. Lower layer powder for a coating type magnetic recording medium comprising the following iron oxyhydroxide powder. 5. Consisting of acicular particles having an average major axis length of 0.01 to 0.5 μm, wherein Al and Si are contained in a total amount of 0.1 to 30% by weight.
A lower layer powder for a coating type magnetic recording medium, comprising an iron oxyhydroxide powder containing and having an amount of H ₂ O released at 100 ° C. of 2% by weight or less. 6. A needle-like particle having an average major axis length of 0.01 to 0.5 μm, a tap density of 0.4 or more and 100 ° C.
Powder for a coating type magnetic recording medium comprising iron oxyhydroxide powder in which the amount of H ₂ O released in step ₂ is 2% by weight or less. 7. It is composed of acicular particles having an average major axis length of 0.01 to 0.5 μm, contains 0.1 to 30% by weight of Al, and has a decomposition temperature in the air of 210 ° C. or higher and 100 ° C. A lower layer powder for a coating type magnetic recording medium comprising an iron oxyhydroxide powder having an amount of H ₂ O to be released of 2% by weight or less.